Although plastics are known to have electrical insulating properties such materials are increasingly required to have a certain electrical conductivity for special applications. In addition to the shielding of electronic components from external electromagnetic fields, (e.g., in the case of computer housings), such applications particularly concern cases in which static electricity is to be discharged, e.g., in the packaging industry, for instance, in the storage of explosives.
Additional applications include integrated circuit components, etc., medical rubber articles having antistatic properties, wall-to-wall carpets having electrostatic properties, antistatic "clean rooms", and electrically conductive metal-joining adhesives. Plastic components which are electrically conductive or provided with an electrically conductive surface film may be provided with an electrostatic paint.
It is known to render polymers electrically conductive by an addition of conductive particles. For instance, metal or carbon black particles, semiconducting oxides, such as zinc oxide, or iodides, such as copper iodide, may be used.
As a rule, polymers containing a commercially available additive as a filler have a black color because the, contain carbon black or metal particles. But a black color is not desired in many cases. Polymers containing, e.g., zinc oxide, as a filler are not stable with respect to their electrical conductivity, and polymers containing, e.g., copper iodide, as a filler are not sufficiently inert. Titanium oxide doped with antimony may be unacceptable toxicologically. From European Patent specification 025 583 it is known to coat titanium oxide particles with a layer consisting of antimony-doped tin oxide. The previously known electrically conductive white powder may be transformed into an electrically conductive color pigment by an addition of dyestuffs or pigments.
The electrical conductivity of copper has also been known for a long time but its fairly high price has opposed its wide use as an antistatic additive. Further, pure copper sulfide is almost black so that the material is very close phenomenologically to much less expensive conductivity-imparting carbon blacks, which are also black.
It is, therefore, an object of the present invention to provide an electrically conductive powder which combines a stable electrical conductivity with a high dispersibility in polymers and resins and which can be produced in a simple and economical manner.